1. Field of the Invention
The present invention relates to a grounded-base transistor amplifier and, more particularly, to a grounded-base transistor amplifier having a dramatically improved noise figure (NF).
2. Description of the Related Art
Transistor amplifiers can be roughly divided into the following three types according to the grounding mode of the amplifying transistors thereof: grounded-emitter amplifiers, grounded-base amplifiers, and grounded-collector (emitter-follower) amplifiers. These three different types of amplifiers exhibit different operating characteristics and are typically used in different applications.
FIG. 5 is a circuit configuration diagram showing an example of a grounded-emitter amplifier. More specifically, FIG. 5 shows a grounded-emitter differential transistor amplifier, which is an example of a grounded-emitter amplifier.
The grounded-emitter differential transistor amplifier shown in FIG. 5 includes differentially-connected transistors 31 and 32, collector load resistors 33 and 34, a pair of transistors 35 and 36 constituting a current mirror circuit (constant-current circuit), a constant current source 37, a signal input terminal 38, a signal output terminal 39, and an operating power supply 40.
The transistors 31 and 32, which are differentially connected, have bases thereof connected to the signal input terminal 38. The transistor 31 has a collector connected to one end of the collector load resistor 33, and the transistor 32 has a collector connected to one end of the collector load resistor 34. The collectors of both transistors 31 and 32 are also connected to the signal output terminal 39. The transistors 31 and 32 also have commonly connected emitters, which are connected to the collector of the transistor 35 of the current mirror circuit. The other end of the collector load resistor 33 and the other end of the collector load resistor 34 are commonly connected to the operating power supply 40. In the current mirror circuit, the bases of the transistors 35 and 36 are commonly connected and the collector of the transistor 36 and one end of the constant current source 37 are connected to the commonly connected bases. The emitters of the transistors 35 and 36 are grounded via resistors. The other end of the constant current source 37 is directly grounded.
In the configuration described above, whenever constant current is supplied from the constant current source 37 to the current mirror circuit, the amount of current flowing through the transistor 36 is set in accordance with the constant current. Further, the current mirror determines the amount of sink current flowing through the transistor 35. Since the collector of the transistor 35 is connected to the commonly connected emitters of the transistors 31 and 32, the sink current flowing through the transistor 35 is divided through the transistors 31 and 32, thereby causing an operating bias current to be applied to the transistors 31 and 32. In the illustrated circuit, an amplification-balanced high frequency signal supplied to the signal input terminal 38 is applied between the transistors 31 and 32 and subjected to differential amplification through the transistors 31 and 32 before it is supplied to the signal output terminal 39 and taken out as an amplified balanced high frequency signal. If an unbalanced signal is supplied to or taken out of the amplifier, then a known unbalanced-to-balanced transformer (not shown) or a balanced-to-unbalanced transformer (not shown) is typically connected to the signal input terminal 38 or the signal output terminal 39.
The noise figure (NF) of a transistor amplifier is normally related to the noise voltage which is generated by transistors or resistors used in the amplifier. More specifically, whenever currents flow through the transistors and resistors, power is consumed and a noise voltage occurs. The magnitude of the noise voltage is directly proportional to the NF of the amplifier.
In the known grounded-emitter differential transistor amplifier shown in FIG. 5, the noise voltage generated by the transistor 35 of the constant-current circuit is supplied to the emitters of the pair of transistors 31 and 32 in the same phase and output from the collectors thereof in the same phase. Hence, no noise voltage appears between the collectors.
Conventional grounded-base differential transistor amplifiers are similar to the above-described grounded-emitter differential transistor amplifiers, with the difference being that the input signals of the grounded-base amplifiers are applied to the emitters of the transistors, and that the bases of the transistors are connected to a common potential. Moreover, grounded-base differential transistor amplifiers are characterized by low input impedance, high output impedance, and good distortion characteristics in response to large inputs.
A problem with conventional grounded-base differential transistor amplifiers is the production of independent noise voltages by a constant-current circuit of the amplifier which are applied out of phase to the emitters of the amplifying transistors. These independent noise voltages are then amplified by the amplifying transistors, thereby causing the grounded-base differential transistor amplifier to exhibit a high NF.